The invention relates to a triggering circuit for safety devices particularly restraint systems, such as air bags or belt tightening devices for motor vehicle occupants.
Prior art triggering circuits of this type have typically included an acceleration sensor and an analyzing circuit. Because the determination of whether to activate the safety device is based on the output signal of the acceleration sensor, quality and dependability of such acceleration sensors are most important. During its whole useful life, the sensor may vary its sensitivity only slightly.
In order to increase the safety of the overall triggering mechanism, it is known, for example, from the German Patent Document DE-PS 34 00 533 to equip the triggering circuit with an additional safety switch in the form of a mechanical acceleration switch connected in series with the switch in the energizing circuit (controlled by the analyzing circuit) of the ignition element for the safety device. The energizing circuit is therefore closed only when two criteria exist simultaneously; that is, when the analyzed acceleration signal exceeds a threshold value, and the safety switch is also closed. The safety switch may, for example, be a small mercury switch which closes a contact in the energizing circuit as a result of the deceleration during an impact of the motor vehicle. A test circuit for the whole triggering device is also disclosed, in which an electric signal is supplied to the analyzing circuit instead of the sensor signal; so that a performance test of the analyzing circuit becomes possible. Triggering of the safety device during the test is not possible because the mechanical safety switch remains open.
A test for an electric or electronic analyzing circuit is also disclosed in German Patent Document DE-OS 28 51 333, in which the triggering of the ignition element of the safety device is performed by a main power switch and an additional auxiliary power switch connected in parallel thereto. During normal operation, the auxiliary power switch is closed so that the safety device can be triggered upon impact; the auxiliary power switched is opened only during the testing so that the main power switch can be closed and the analyzing circuit can be tested without the risk of activating the safety device.
Since it cannot be closed from the outside, the operability of the mechanical safety switch of the triggering circuit according to the above mentioned German Patent Document De-PS 34 00 533 must be tested only indirectly by determining whether the electric resistance is sufficiently high when the switch is opened. Since it cannot be tested whether the safety switch actually closes, a safety switch that cannot close may go undetected, and the safety device cannot be activated in an emergency.
Moreover, mechanical safety switches cannot be installed in the motor vehicle in an arbitrary position. Mechanical safety switches such as the above-mentioned mercury switches, for example, use the gravitation of the earth as a restoring force. In the case of modern safety devices, however, all components are integrated in the smallest possible space and (in the case of an air bag) the whole safety device is installed in the steering wheel of the motor vehicle. A triggering circuit having a safety switch that must be restored by the earth's gravitation cannot be used for this purpose because, as a result of the turning of the steering wheel, the position of the safety switch changes constantly (also in the off-position of the motor vehicle) so that gravitation restoration is not possible.
It is an object of the present invention to ensure the reliability of the triggering circuit, even in the above-mentioned case. Another object of the invention to provide a safety device which operates reliably independently of the installation of the triggering circuit and which can be tested with respect to operatability in all its components.
These and other objects and advantages are achieved according to the invention by means of a parallel arrangement of at least two acceleration sensors with mutually independent triggering paths. A mechanical safety switch is not required in the case of an AND-operation of the respective triggering paths because the second triggering path takes over the function of the safety switch. The acceleration sensors are installed approximately at the same point in the motor vehicle so that, in the case of an accident, they are affected essentially by the same forces or decelerations. Preferably, direction-sensitive acceleration sensors are used, which have axes of sensitivity situated in the same direction. At least one of the analyzing circuits operates preferably digitally and contains, for example, a microprocessor, while another analyzing circuit operates in an analog mode. For this purpose, for example, the output signals of an analog acceleration sensor, such as a bridge circuit or a piezosensor, are integrated in an integrating circuit and, by way of a threshold value switch, are led to the switch step for the ignition element of the safety device. Upon impact of the motor vehicle, that triggering path which takes over the function of the safety switch is closed earlier, so that a critical threshold for activation of the safety device which is given by another triggering path is definitely observed.
Another advantage of a triggering circuit according to the invention is that a change of sensitivity of an acceleration sensor during the operating time is detected immediately because both acceleration sensors must detect the same driving or interference noises at the same installation point in the motor vehicle. Thus, a test may be performed by means of a microprocessor which is normally present in the analyzing circuit. If only two acceleration sensors are used and their measured values differ from one another beyond an acceptable tolerance threshold, the reliable conclusion is that there is a defect within the sensor arrangement.
In the case of the purely electronic embodiment of a triggering circuit according to the invention, it is also possible to test the operability of all triggering paths, including the associated electronic switch steps, preferably by means of a pulse generator which sequentially emits test pulses to the acceleration sensor which pass through the associated analyzing circuits. The pulse generator, such as a flip-flop arrangement, is controlled by a microprocessor which itself is used as an analyzing circuit for at least one acceleration sensor. The test pulses also pass through the acceleration sensors so that their performance can also be tested. The test pulses, which are generated by the microprocessor or by another test circuit controlling the pulse generator, can control the pulse generation for the generating of a first test pulse only if the whole triggering circuit has been set back previously, that is, if there was a preceding system reset. In this manner, for example, the analog-operating trigger path cannot be energized accidentally if there is a disturbance in the microprocessor program. This is possible only during the testing program. The whole final ignition step for the safety device in this manner, after a self-test, can only be controlled when all (as a rule, both) acceleration sensors are mechanically excited.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.